Extension data handling

ABSTRACT

A length indicator ( 23 ) associated with an encoded representation ( 20 ) of a slice ( 3 ) is parsed. The length indicator ( 23 ) is indicative of a length of an extension field ( 24 ) present in a slice header ( 21 ) of the encoded representation ( 20 ). A decoder ( 40 ) can then determine to ignore, during decoding of the encoded representation ( 20 ), any value of the extension field ( 24 ) in the slice header ( 21 ) where these values are identified based on the length indicator ( 23 ). As a consequence, extension fields ( 23 ) can be added to slice headers ( 21 ) and still enable legacy decoders ( 40 ) to correctly decode the encoded representations ( 20 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. §371 National Phase Entry Applicationfrom PCT/SE2012/051477, filed Dec. 21, 2012, and designating the UnitedStates, and claims priority to Provisional Application No. 61/623,290,filed Apr. 12, 2012. The above-mentioned applications are incorporatedby reference herein.

TECHNICAL FIELD

The embodiments generally relate to encoding and decoding of slices, andin particular to handling extension data in connection with encoding anddecoding of slices.

BACKGROUND

High Efficiency Video Coding (HEVC) is a new video coding standardcurrently being developed in Joint Collaborative Team-Video Coding(JCT-VC). JCT-VC is a collaborative project between Moving PictureExperts Group (MPEG) and International Telecommunication UnionTelecommunication standardization sector (ITU-T). Currently, a CommitteeDraft (CD) is defined that includes a number of new tools and isconsiderably more efficient than H.264 Advanced Video Coding (AVC).

It is likely that extensions of HEVC will be defined in the future, suchas a multi-view extension and a scalable extension. These extensionscould then contain data relating to, for instance, additional viewsdepending on a base view or additional layers depending on a base layer.

It can be assumed that backwards compatibility for some parts of anencoded sequence, e.g. the base view or the base layer, is arequirement. However, it is important that a legacy, i.e. basespecification compliant, decoder is able to correctly handle thebitstream even when using such extensions.

HEVC defines some Network Abstraction Layer (NAL) unit types to bereserved for, among others, extensions. Also parameter sets, e.g.Sequence Parameter Set (SPS), Picture Parameter Set (PPS) and AdaptationParameter Set (APS), contain extension fields at the end of their syntaxtables. It is specified that a decoder conforming to the basespecification of HEVC shall ignore the data in the extension field atthe end of a parameter set. The syntax for the extension data is shownbelow in an illustrative example in the form of SPS as parameter set.The syntax would be similar if using other parameter sets than SPS. Thetotal length of a parameter set is known by the decoder through thesyntax element NumBytesInRBSP, typically provided by the system layer,so the decoder will have no problems in detecting where the extensionfield ends.

seq_parameter_set_rbsp( ) { Descriptor . . . sps_extension_flag u(1) if(sps_extension_flag ) white( more_rbsp_data( ) ) sps_extension_data_flagu(1) rbsp_trailing_bits( ) }

A NAL unit that contains an encoded slice of a picture consists of twoparts: first the slice header and then the slice data. There iscurrently no extension field for encoded slices.

The slice header contains information about the current slice, e.g.slice type, which reference pictures to be used, etc. There is a needfor adding extensions and extension fields in HEVC. However, in such acase it is important that both a legacy decoder conforming to the basespecification, i.e. not using extensions, and an extension-compliantdecoder could correctly handle a bitstream comprising such extensions.

The current layout of a slice header for an encoded representation of aslice makes this need hard or even impossible to implement when havingboth legacy and extension-compliant decoders.

SUMMARY

It is a general objective to enable usage of slice header extensions.

It is a particular objective to handle slice header extensions inconnection with encoding and decoding of slices.

These and other objectives are met by embodiments disclosed herein.

An aspect of the embodiments relates to a method of decoding an encodedrepresentation of a slice of a picture. The encoded representationcomprises a slice header and slice data. The method comprises parsing alength indicator associated with the encoded representation andindicative of a length of an extension field in the slice header. Themethod also comprises determining to ignore, during decoding of theencoded representation, any value of the extension field identifiedbased on the length indicator.

A related aspect of the embodiments defines a decoder for decoding anencoded representation of a slice of a picture. The encodedrepresentation comprises a slice header and slice data. The decodercomprises a parsing unit configured to parse a length indicatorassociated with the encoded representation and indicative of a length ofan extension field in the slice header. The decoder also comprises aprocessing unit configured to cause the decoder to ignore, duringdecoding of the encoded representation, any value of the extension fieldidentified based on the length indicator.

Another aspect of the embodiments relates to a method of decoding anencoded representation of a slice of a picture. The encodedrepresentation comprises a slice header and slice data. The methodcomprises parsing a length indicator associated with the encodedrepresentation and indicative of a length of an extension field in theslice header. The method also comprises identifying the extension fieldin the slice header based on the length indicator. The encodedrepresentation of the slice is decoded based on at least one value ofthe extension field identified based on the length indicator.

Another related aspect of the embodiments defines a decoder for decodingan encoded representation of a slice of a picture. The encodedrepresentation comprises a slice header and slice data. The decodercomprises a parsing unit configured to parse a length indicatorassociated with the encoded representation and indicative of a length ofan extension field in the slice header. The decoder also comprises anextension field identifying unit configured to identify the extensionfield in the slice header based on the length indicator. A decoding unitof the decoder is configured to decode the encoded representation of theslice based on at least one value of the extension field identified bythe extension field identifying unit.

A further aspect of the embodiment relates to a method of encoding aslice of a picture. The method comprises generating a length indicatorindicative of a length of an extension field present in a slice headerof the slice. The slice is encoded into an encoded representation of theslice comprising the slice header and slice data. The length indicatoris associated with the encoded representation.

A related further aspect of the embodiments defines an encoder forencoding a slice of a picture. The encoder comprises a generating unitconfigured to generate a length indicator indicative of a length of anextension field present in a slice header of the slice. An encoding unitis configured to encode the slice into an encoded representation of theslice comprising the slice header and slice data. The encoder alsocomprises an associating unit configured to associate the lengthindicator with the encoded representation.

An additional aspect of the embodiments relates to a receiver comprisinga decoder for decoding an encoded representation of a slice of apicture. The encoded representation comprises a slice header and slicedata. The decoder comprises a parsing unit configured to parse a lengthindicator associated with the encoded representation and indicative of alength of an extension field in the slice header. The decoder alsocomprises a processing unit configured to cause the decoder to ignore,during, decoding of the encoded representation, any value of theextension field identified based on the length indicator.

Another additional aspect of the embodiments relates to a receivercomprising a decoder for decoding an encoded representation of a sliceof a picture. The encoded representation comprises a slice header andslice data. The decoder comprises a parsing unit configured to parse alength indicator associated with the encoded representation andindicative of a length of an extension field in the slice header. Thedecoder also comprises an extension field identifying unit configured toidentify the extension field in the slice header based on the lengthindicator. A decoding unit of the decoder is configured to decode theencoded representation of the slice based on at least one value of theextension field identified by the extension field identifying unit.

Yet another additional aspect of the embodiments relates to atransmitter comprising an encoder for encoding a slice of a picture. Theencoder comprises a generating unit configured to generate a lengthindicator indicative of a length of an extension field present in aslice header of the slice. The encoder also comprises an encoding unitconfigured to encode the slice into an encoded representation of theslice comprising the slice header and slice data. An associating unit ofthe encoder is configured to associate the length indicator with theencoded representation.

A further aspect of the embodiment relates to a computer program fordecoding an encoded representation of a slice of a picture. The encodedrepresentation comprises a slice header and slice data. The computerprogram comprises code means which when run by a processor causes theprocessor to parse a length indicator associated with the encodedrepresentation and indicative of a length of an extension field in theslice header. The processor is also caused to determine to ignore,during decoding of the encoded representation, any value of saidextension field identified based on the length indicator.

A further related aspect of the embodiment defines a computer programfor decoding an encoded representation of a slice of a picture. Theencoded representation comprises a slice header and slice data. Thecomputer program comprises code means which when run by a processorcauses the processor to parse a length indicator associated with theencoded representation and indicative of a length of an extension fieldin the slice header. The processor is also caused to identify, based onthe length indicator, the extension field in said slice header anddecode the encoded representation of the slice based on at least onevalue of the extension field identified based on the length indicator.

Yet a further related aspect of the embodiments defines a computerprogram for encoding a slice of a picture. The computer programcomprises code means which when run by a processor causes the processorto generate a length indicator indicative of a length of an extensionfield present in a slice header of the slice. The processor is alsocaused to encode the slice into an encoded representation of the slicecomprising the slice header and slice data. The processor is furthercaused to associate the length indicator with the encodedrepresentation.

Another additional aspect of the embodiments relates to a computerprogram product comprising computer readable medium and a computerprogram stored on the computer readable medium.

The present embodiments enable introduction of extension fields intoslice headers of encoded representations of slices and still producingsuch encoded representations that could be correctly handled even bylegacy decoders that are not compatible with extension fields.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by making reference to the following descriptiontaken together with the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a video stream of picturescomprising one or more slices;

FIG. 2 is an illustration of a data packet comprising a NAL unit;

FIGS. 3A-3C are illustrations of encoded representations of a slice;

FIG. 4 is a flow diagram of a method of decoding an encodedrepresentation of a slice according to an embodiment;

FIG. 5 is a flow diagram illustrating additional, optional steps of themethod in FIG. 4 according to an embodiment;

FIG. 6 is a flow diagram illustrating additional, optional steps of themethod in FIG. 4 according to another embodiment;

FIG. 7 is a flow diagram illustrating additional, optional steps of themethod in FIG. 4 according to a further embodiment;

FIG. 8 is a flow diagram of a method of decoding an encodedrepresentation of a slice according to another embodiment;

FIG. 9 is a flow diagram of a method of encoding a slice according to anembodiment;

FIG. 10 is a flow diagram illustrating additional, optional steps of themethod in FIG. 9 according to an embodiment;

FIG. 11 is a flow diagram illustrating additional, optional steps of themethod in FIG. 9 according to another embodiment;

FIG. 12 is a flow diagram illustrating an embodiment of the associatingstep in FIG. 11;

FIG. 13 is a schematic block diagram of a decoder according to anembodiment;

FIG. 14 is a schematic block diagram of a decoder according to anotherembodiment;

FIG. 15 is a schematic block diagram of a decoder according to a furtherembodiment;

FIG. 16 is a schematic block diagram of a receiver according to anembodiment;

FIG. 17 is a schematic block diagram of an encoder according to anembodiment;

FIG. 18 is a schematic block diagram of an encoder according to anotherembodiment;

FIG. 19 is a schematic block diagram of a transmitter according to anembodiment; and

FIG. 20 is a schematic block diagram of a computer comprising a computerreadable medium and a computer program according to an embodiment.

DETAILED DESCRIPTION

Throughout the drawings, the same reference numbers are used for similaror corresponding elements.

The present embodiments are directed towards extension data handling inconnection with encoding and decoding of slices of pictures. Hence, theembodiments enable introduction and usage of so called extension fields,also denoted extension syntax elements, in slice headers for the slices.Such extension fields can, according to the embodiments, be added to theslice headers and still present an encoded representation that could becorrectly handled both by extension-compliant decoders and thosedecoders that cannot handle extension data, i.e. are conformant to aprofile that does not use extension fields. These decoders are generallydenoted legacy decoders or base specification compliant decoders.

The presence of both extension-compliant and legacy decoders imposesproblems in connection with usage of extension fields in the sliceheaders. Hence, a legacy decoder needs to be able to correctly decodeand interpret the data of an encoded representation of a slice even ifthe encoded representation carries extension data in its slice header.

It is not possible to add the same kind of extension field as in theparameter sets discussed in the background section, where all additionaldata at the end of the syntax structure is ignored by the decoder. Thereason being that the decoder has no way to identify where the sliceheader ends and the slice data begins. Thus, the length of theindividual data structures, i.e. slice header and slice data, are notknown, only the total length by the syntax element NumBytesInRBSP. Thus,the slice layer raw byte sequence payload (RBSP) syntax generally lookslike:

slice_layer_rbsp( ) { slice_header( ) slice_data( )rbsp_slice_trailing_bits( ) }

Thus, it is not possible to add a simple extension field where alladditional data at the end of the slice header is ignored by a legacydecoder since there would be no way for the legacy decoder to identifywhere the slice header ends and the slice data begins.

It is proposed to associate a length indicator with the encodedrepresentation of the slice and where this length indicator isindicative of a length of an extension field present in the slice headerof the slice. Hence, the length indicator enables a decoder to identifythe portion of the encoded representation of the slice and the sliceheader that constitutes the extension field. Thus, the legacy decodercould therefore determine which portion of the encoded representation toignore during decoding since it contains extension data that the legacydecoder cannot handle.

However, whereas a legacy decoder should ignore the extension dataduring decoding an extension-compliant decoder is able to decode and usethis extension data during the decoding.

FIG. 1 is a schematic illustration of a video sequence or stream 1 ofpictures 2. As is well known in the art each picture 2 can comprise oneor more so called slices 3. Thus, each picture 2 of the video stream 1is divided into one or more slices 3, where each slice 3 is anindependently decodable piece of the picture 2. In other words, if oneslice 3 is lost, the other slices 3 of that picture 2 are stilldecodable.

The encoding of a slice 3 generates an encoded representation of theslice 3 comprising a slice header and slice data. The encodedpresentation is output from the encoding process as a so called NetworkAdaptation Layer (NAL) unit 11 as shown in FIG. 2. The first part of theNAL unit 11 is a header that contains an indication of the type of datain the NAL unit. The remaining part of the NAL unit 11 contains payloaddata in the form of the slice header and slice data.

The NAL unit 11 may then be added with headers 12 to form a data packet10 that can be transmitted as a part of a bistream from the encoder tothe decoder. For instance, Real-time Transport Protocol (RTP), UserDatagram Protocol (UDP) and Internet Protocol (IP) headers 12 could beadded to the NAL unit 11. This form of packetization of NAL units 11merely constitutes an example in connection with video transport. Otherapproaches of handling NAL units 11, such as file format, MPEG-2transport streams, MPEG-2 program streams, etc. are possible.

The embodiments as disclosed herein are in particular applicable anddirected towards decoding and encoding of slices 3 of pictures 2 in avideo stream or sequence 1, where the video stream or sequence 1comprises multiple pictures 2 as shown in FIG. 1. The embodiments willtherefore mainly be discussed in connection with such video decoding andcoding.

However, the embodiments could also be applied to decoding and encodingof slices in a single picture, e.g. in connection with such decoding andencoding of still pictures or images. For instance, extension data couldbe used in connection with spatially scalable still pictures.

FIG. 4 is a flow diagram illustrating a method of decoding an encodedrepresentation of a slice of a picture, optionally but preferably in avideo stream. The method of FIG. 4 is preferably a method performed by adecoder during decoding of an encoded representation of a slice of apicture or of a decoding method for an encoded representation of a sliceof a picture.

FIGS. 3A-3C illustrate non-limiting examples of such an encodedrepresentation 20 of a slice. The encoded representation 20 comprises aslice header 21 and slice data 25.

The method of FIG. 4 comprises a step S1 in which a length indicator(LI) 23 associated with or to the encoded representation 20 is parsed.This length indicator 23 is indicative of a length of an extension field(EXT F) 24 in the slice header 21. In a following step S2 it isdetermined to ignore, during decoding of the encoded representation 20,any value of the extension field 24 identified based on the lengthindicator 23.

This means that the length indicator 23 associated with the encodedrepresentation 20 can be used to identify the portion of the encodedrepresentation 20 occupied by the extension field 24. Hence, a decodercan thereby use the length indicator 23 to determine or decide to ignoreand thereby not use value(s) or data of the extension field 24, wherethis extension field 24 is identified in the slice header based on thelength indicator 23. Thus, the decoder thereby uses the length indicator23 to skip an amount or a portion of data in the slice header 21, wherethe amount or portion of data skipped is indicated by the lengthindicator 23.

In a particular embodiment, the method of decoding an encodedrepresentation 20 of a slice of a picture, such as in a video stream,comprises parsing a length indicator 23 associated with the encodedrepresentation 20 and indicative of a length of an extension field 24 inthe slice header 21. The method further comprises in this particularembodiment ignoring, during decoding of the encoded representation 20,any value of the extension field 24 identified based on the lengthindicator 23.

The length indicator 23 associated with the encoded representation 20enables also legacy decoders to handle and decode the encodedrepresentation 20. These legacy decoders simply determine to ignore thevalue(s) or data obtained from the portion of the slice header 21 thatis occupied by the extension field 24 and where this portion of theslice header 21 is identified based on the parsed length indicator 23.

Associating as used herein implies that it is possible to identify andretrieve the relevant length indicator 23 based on the encodedrepresentation 20. As is further disclosed herein this could, forinstance, be achieved by including the length indicator 23 in theencoded representation 20, typically in the slice header 21, see FIGS.3A and 3B, or by including data, such as an identifier 26, in theencoded representation 20 and where this data or identifier 26 can beused to identify the relevant length indicator 23, see FIG. 3C. In thislatter case, the length indicator 23 could be provided in another datastructure than the encoded representation 20. The data or identifier 26therefore points to and enables identification of this other datastructure.

FIG. 5 is a flow diagram illustrating additional steps of the methodaccording to a particular embodiment. These additional, but optional,steps start with step S10 that continues from step S1 in FIG. 4. In stepS10 a length of the extension field is determined based on the lengthindicator 23 parsed in step S1. In a next step S11 the end of theextension field 24 in the slice header 21 is identified based on thelength determined in step S10. In the following step S2 the decoder cantherefore decide or determine to ignore the value or values of theextension field 24 up to the end of the extension field 24 as identifiedin step S11.

Optionally the start of the extension field 24 in the slice header 21 isalso identified. In an embodiment, the start could be in a fixed orpredefined position in the slice header 21, such as a defined bit orbyte position in the slice header 21. Alternatively, the start of theextension field 24 could be determined based on the length indicator 23,such as directly following the length indicator 23 in the slice header21, see FIGS. 3A and 3B. Generally, a position of a syntax element inthe slice header 21 is given by a length of preceding syntax elements.This is in particular true if variable length coding is applied to oneor more preceding syntax elements in the slice header 21. In such acase, the decoder typically starts parsing and decoding the slice header21 from the first bit position and going through the various syntaxelements until it reaches to the start of the extension field 24.

In an embodiment a length indicator 23 indicating the length ofadditional slice data, i.e. the extension field 24, is introduced. Thelength indicator 23 is exemplified by a syntax element in e.g. the sliceheader 21 or another data structure, which is associated with the sliceheader 21, or the length may derived by a bit-pattern. Accordingly, anencoder is configured to use the defined syntax element 23 to signal thelength of the additional data. A decoder is configured to determine thelength of the additional data. The additional data will be ignored bydecoders conformant to some profiles while decoders conformant to otherprofiles will parse it and use the values of syntax element in theadditional data. Decoders conformant to profiles that ignores it willuse the length information to determine the end of the slice header 21such that the NAL unit can be decoded.

The embodiments therefore enable extension of a slice header 21 in asimple and bitrate efficient way.

In a particular embodiment applicable to the method of FIG. 4 and to theembodiment shown in FIG. 5, the length indicator 23 is a codeword orsyntax element present in the slice header 21 as shown in FIGS. 3A and3B. This means that step S1 then comprises parsing the length indicator23 present in the slice header 21.

In a particular embodiment, the length indicator 23 is present in theslice header 21 directly preceding the extension field 24. Hence theextension field 24 directly follows the length indicator 23 in the sliceheader 21 in this particular embodiment. In such a case, step S2 of FIG.4 could comprise, in an embodiment, determining to ignore, duringdecoding of the encoded representation 20, any value or data of theextension field 24 directly following the length indicator 23 in theslice header 21 up to an end of the extension field 24. This end of theextension field 24 is identified based on the length indicator 23, suchas disclosed above in connection with step S11 of FIG. 5.

In another particular embodiment, the length indicator 23 is notnecessarily present in the slice header 21 but otherwise associated withthe encoded representation 20. For instance, a slice header 21 typicallycomprises an identifier 26 of a parameter set associated with theencoded representation 20 and comprising data applicable to the presentslice. FIG. 3C illustrates this case with a parameter set (PS)identifier (ID) 26 in the slice header 21.

For instance, an adaptation parameter set (APS) comprises controlinformation valid for more than one slice. The control information maydiffer between the slices. A picture parameter set (PPS) comprisescontrol information valid for several pictures, and which may be thesame for multiple pictures of the same video sequence or stream. Asequence parameter set (SPS) comprises control information valid for anentire video sequence or stream. HEVC also uses so called a VideoParameter Set (VPS). In such a case, the length indicator 23 could bepresent in any of these parameter sets, such as in the APS, VPS, PPS orSPS applicable to the present slice. In such a case, the slice header 21preferably comprises a parameter set identifier 26 enablingidentification of the relevant parameter set that carries the lengthindicator 23. The decoder could then use this parameter set identifier26 in order to identify the correct parameter set and then parse thelength indicator 23 from the identified parameter set. The parameter setidentifier 26 could directly identify the parameter set, such as an APSidentifier or a PPS identifier. In an alternative approach the parameterset identifier 26 identifies a parameter set that in turn comprisesanother parameter set identifier to another parameter set that carriesthe length indicator 23. For instance, the slice header 21 couldcomprise a PPS identifier to a PPS that is applicable to the presentslice. This PPS may in turn comprise an SPS identifier to an SPS that isapplicable to the PPS and the present slice and where this SPS comprisesthe length indicator 23.

If the size or length of the extension field 24 in terms of the numberof bytes or bits is likely to be similar for several slices in the videostream it could be preferred to add the length indicator 23 in aparameter set instead of signaling it directly in each encodedrepresentation 20. In such a case, the total size or length of theencoded representation 20 could be somewhat reduced by not having toinclude the bits of the length indicator 23 in the slice header 21. Inthis approach multiple slices will therefore share a same lengthindicator 23 by referring to the same parameter set that comprises thelength indicator 23.

However, there is generally a much larger flexibility in including thelength indictor 23 directly in the slice headers 21 in each encodedrepresentation 20. As a consequence, the size or length of the extensionfield 24 may therefore be allowed to differ between different slices. Inaddition, no referral to a parameter set is required in order to parsethe length indicator 23.

The parsing of the length indicator as shown in step S1 in FIG. 4 doesnot necessarily have to be performed for each encoded representation 20.In particular, if the length indicator 23 is present in a parameter setidentified based on a parameter set identifier 26 present in the sliceheader the relevant parameter set could be identified for the firstencoded representation 20 of the video sequence comprising the parameterset identifier 26. The data in the identified parameter set is thenparsed and the decoded values and data present therein, including thelength indicator 23, are then typically stored in a memory of thedecoder. Then the decoder keeps track of the activated parameter set,i.e. a parameter set that has already been parsed and decoded. Thismeans that when a subsequent encoded representation 20 of the videosequence comprises a parameter set identifier 26 identifying thisparameter set the data, including the length indicator 23, could beretrieved from the stored values in the memory.

Hence, parsing the length indicator 23 in step S1 does not necessarilyhave to be performed for each encoded representation 20, in particularif the length indicator 23 is present in a parameter set. However, thisstep S1 is preferably performed at least once for an encodedrepresentation 20 having a parameter set identifier 26 pointing to therelevant parameter set in its slice header 21. Step S1 could then, forany following encoded representation 20 of the video sequence comprisinga parameter set identifier 26 for this relevant parameter set, comprisereading or retrieving, from a memory, the length indicator associatedwith the encoded representation and indicative of the length of theextension field in the slice header.

The length indicator 23, see FIGS. 3A and 3B, or the parameter setidentifier 26, see FIG. 3C, does not necessarily have to directlyprecede the extension field 24 in the slice header 21 as is shown inFIGS. 3A-3C. It is generally preferred to have the extension field 24directly following the length indicator 23 or the parameter setidentifier 26. However, this is not necessary. In alternativeembodiments, other slice header data, i.e. at least one syntax elementor codeword, could be present in between the position of the lengthindicator 23 or the parameter set identifier 26 and the extension field24 in the slice header 21.

FIG. 6 is a flow diagram of additional, optional steps of the method ofFIG. 4. In step S21 a slice header extension present flag associatedwith the encoded representation is parsed. This slice header extensionpresent flag could be present in the slice header 21 or otherwiseassociated with the encoded representation 20. For instance, the sliceheader extension present flag could be present in a parameter setassociated with the slice, such as in a PPS. The method then preferablycomprises the optional step S20 in which a parameter set, such as a PPS,associated with the encoded representation 20 is identified based on aparameter set identifier, such as a PPS identifier, obtainable based onthe slice header 21. The parameter set identifier could be directlyretrieved from the slice header 21 or could be retrieved from anotherparameter set, which is identified by another parameter set identifierdirectly retrieved from the slice header 21. Step S21 then preferablycomprises parsing the slice header extension flag present in theparameter set, such as in the PPS.

Parsing the slice header extension present flag in step S21 does notnecessarily have to be performed for every encoded representation 20 ofa video sequence, in particular if the slice header extension presentflag is present in a parameter set as identified in step S20. In such acase, the relevant parameter set could be identified, parsed and decodedfor the first encoded representation 20 having a parameter setidentifier 26 pointing to this parameter set. The decoded data of theparameter set, including the slice header extension present flag, couldthen be stored in a memory of the decoder. In such a case, the value ofthe slice header extension present flag can simply be read or retrievedfrom a memory for any subsequent encoded representation 20 having aparameter set identifier 26 pointing to the previously parsed anddecoded parameter set.

The slice header extension present flag indicates whether there is anyextension field 24 present or not in the slice header 21. Thus, if theslice header extension present flag has a first predefined value, suchas 1_(bin), an extension field 24 is present in the slice header 21 andthe encoded representation 20 is associated with a length indicator 23.However, if the slice header extension present flag has a secondpredefined value, such as 0_(bin), no extension field 24 is present inthe slice header 21 and therefore no length indicator 23 is associatedwith the encoded representation 20.

In a next step S22 the parsed slice header extension present flag isanalyzed and it is determined whether it has the first predefined valueor the second predefined value. For instance, if the slice headerextension present flag is set, i.e. equal to 1_(bin), the methodcontinues to step S1 of FIG. 4 where the length indicator 23 is parsed.Thus, in this case an extension field 24 is present and a legacy decoderneeds to identify the portion of the slice header 21 that correspond tothe extension field 24.

If the slice header extension present flag is not set, i.e. equal to0_(bin), the method instead continues to step S23 where the decoder cansimply start parsing and decoding the slice data 25 that follows theslice header 21. Thus, in this case there is no extension field presentin the slice header 21 that needs to be identified by a legacy decoder.

In a particular embodiment the picture parameter set RBSP syntax couldlook like:

pic_parameter_set_rbsp( ) { Descriptor . . .slice_header_extension_present_flag u(1) . . . }

slice_header_extension_present_flag equal to 0 specifies that no sliceheader extension syntax elements are present in the slice header forcoded pictures referring to the picture parameter set.

A general slice header syntax could then in this particular embodimentlook like:

slice_header( ) { Descriptor . . . if(slice_header_extension_present_flag ) { slice_header_extension_lengthue(v) for( i=0; i<slice_header_extension_length; i++ )slice_header_extension_data_byte[ i ] u(8) } byte_alignment( ) }

slice_header_extension_length specifies the length of the slice headerextension data in bytes, not including the bits used for signalingslice_header_extension_length itself. The value ofslice_header_extension_length could be in the range of 0 to 256,inclusive. The size of slice_header_extension_length does notnecessarily have to be in Universal Variable Length Code (UVLC) butcould of fixed length, i.e. u(q) for some positive integer q.

slice_header_extension_data_byte may have any value. Legacy decodersshould ignore the value of slice_header_extension_data_byte.slice_header_extension_data_byte could alternative use bits, i.e. u(1),instead of bytes, i.e. u(8).

The slice header extension present flag implies that the decoder candetermine, by simply reading this single flag, preferably in the form ofan one-bit flag, whether an encoded representation 20 comprises anyextension field 24 that needs to be processed, i.e. parsed and used byan extension-compliant decoder or ignored by a legacy decoder. Hence,usage of the slice header extension present flag simplifies decoding ofthe encoded representations 20 in situations where some encodedrepresentations 20 could comprise extension fields 24 whereas others donot. In such a case, the decoder does not need to parse and use anylength indicator 23 but can simply investigate the value of the sliceheader extension present flag.

The slice header extension present flag is advantageously inserted in aparameter set since it is likely that multiple slices in the videostream will either have no extension fields 24 or have extension fields24. It is then more bit-efficient to signal the slice header extensionpresent flag in the parameter set as compared to including such a sliceheader extension present flag in each slice header 21.

In a particular embodiment, the slice data 25 starts directly followingthe end of the extension field 24 in the encoded representation 20.Hence, in this approach the bit position following the end of theextension field 24 as identified based on the length indicator 23constitutes the first bit of the slice data 25.

In another particular embodiment, byte alignment data 22 could beinserted between the extension field 24 and the slice data 25 asindicated in FIG. 3A and further mentioned in the general slice headersyntax above. Such byte alignment data may be employed at the end of theslice header 21 to get the start of the slice data 25 byte aligned. Insuch a case, the length indicator 23 can be used to identify the portionof the slice header 21 that is occupied by the extension field 24 andcould, for instance, indicate the length of the extension field 24 andthereby signal the end of the extension field 24. Thereafter follows thebyte alignment data 22 and then the slice data 25. This means that evenif byte alignment data 22 is used the decoder will be able to identifythe start of the slice data 25 and the portion of the slice header 21that constitutes the extension field 24 based on the length indicator23.

The byte alignment data 22 preferably has a well-defined syntax, forinstance:

byte_alignment( ) { Descriptor bit_equal_to_one /* equal to 1 */ f(1)while( !byte_aligned( ) ) bit_equal_to_zero /* equal to 0 */ f(1) }

In this example the byte alignment data always comprises at least onebit. If the end of the extension field 24 should be byte aligned an8-bit codeword, 1000 0000_(bin), is used.

FIG. 3B illustrates an embodiment of the encoded representation 20 of aslice showing that the length indicator 23 and the extension field 24not necessarily has form the last syntax elements (excluding theoptional byte alignment data 22) of the slice header 21. In clearcontrast, the length indicator 23 and the extension field 24, FIG. 3B,or indeed the parameter set identifier 26 and the extension field, FIG.3C, could be present anywhere in the slice header 21.

FIG. 7 is a flow diagram illustrating additional, optional steps of themethod in FIG. 4. The method continues from step S2 in FIG. 4. In a nextstep S30 a start of the slice data 25 in the encoded representation 20is identified based on an end of the extension field 24 as identifiedbased on the length indicator 23, such as disclosed in step S11 of FIG.5. As mentioned above, the start of the slice data 25 could directlyfollow the end of the extension field 24. In an alternative approach,byte alignment data 22 is interposed between the extension field 24 andthe start of the slice data 25. In such a case, the length indicator 23can be used to identify the end of the extension field 24. The followingbit or bits in the encoded representation 20 constitute(s) the bytealignment data 22. This portion of the encoded representation 20 isinterpreted by the decoder as byte alignment data 22 since it has awell-defined syntax, such as exemplified above. Once the end of the bytealignment data 22 is reached the start of the slice data 25 is found. Itis also possible, which is further described below, that other sliceheader data could follow the extension field 24.

When the decoder has identified the start of the slice data 25 it canparse and decode the slice data 25 in step S31 from the identified startof the slice data 25.

A legacy decoder preferably parses and decodes the slice data in stepS31 by ignoring, i.e. not using, any value or data of the extensionfield 24 in the slice header 21 during the parsing and decoding processof step S31. Hence, the legacy decoder cannot use the extension field 24in the slice header 21 since this extension field 24 is, according thebase profile or base specification to which the legacy decoder conforms,is unspecified.

However, an extension-compliant decoder should make use of the data inthe extension field 24 in the slice header during the parsing anddecoding process, optionally including post-decoding processing of theslice data 25.

The present embodiments may also be applied to the case where the sliceheader 21 comprises multiple, i.e. at least two, extension fields 24. Insuch a case, each extension field 24 preferably comprises a respectivelength indicator 23. The extension fields 24 could all be arranged inconnection with the end of the slice header 21, possibly followed byoptional byte alignment data 22. However, it is also possible to have adistributed organization of the extension fields 24 in the slice header21. In this latter case, the extension fields 24 could be providedanywhere in the slice header 21. For instance, a first extension field24 could be present in the first or middle part of the slice header 21,whereas a second extension field 24 could be present in the end part ofthe slice header 21.

If multiple extension fields 24 may be present in the slice header 21the respective length indicators 23 could be present in one or moreparameter sets, in the slice header 21 or be distributed among the sliceheader 21 and one or more parameter sets. For instance, a first lengthindicator 23 of a first extension field 24 could be present in aparameter set, such a PPS. A second length indicator 23 of a secondextension field 24 could then be present in the slice header 21. Thepresence of any of the extension fields 24 and length indicators 23could be signaled by a flag, for instance in a parameter set. Forexample, a PPS could comprise the first length indicator 23 and a flagindicating the presence or absence of the second extension field 24. Thesecond length indicator 23 and the second extension field 24 are then,if the flag has been set to a defined value, present in the slice header21.

In the case of multiple extension fields 24, step S1 of FIG. 4 ispreferably performed once for each extension field 24 and lengthindicator 23. Thus, each length indicator 23 associated with the encodedrepresentation 20 is identified and parsed in order to enableidentification of the respective extension field 24 in the slice header21 so that a legacy decoder can determine to ignore the respective valueor values of the extension fields in step S2.

FIG. 8 is a flow diagram illustrating another embodiment of a method ofdecoding an encoded representation of a slice of a picture. The encodedrepresentation comprises a slice header and slice data. This embodimentis particularly performed by an extension-compliant decoder, i.e. adecoder that understands and can make use of the value or values in anextension field present in the slice header.

The method starts in step S80 where a length indicator associated withthe encoded representation and indicative of a length of an extensionfield in the slice header is parsed. This step S1 basically correspondsto step S1 in FIG. 4 and is not further discussed herein.

A next step S81 identifies the extension field in the slice header basedon the length indicator parsed in step S80. Thus, the length indicatoris used in step S81 in order to identify the portion of the slice headerthat is occupied by the extension field. This step S81 can be performedas previously disclosed herein, for instance in connection with FIGS. 4and 5.

If the slice header comprises multiple extension fields steps S80 andS81 are preferably performed once for each extension field andassociated length indicator.

In the following step S82 the encoded representation of the slice isdecoded based on at least one value, preferably all values, of theextension field in the slice header as identified in step S81 based onthe length indicator parsed in step S80.

Thus, in this embodiment the decoder is extension-compliant and willtherefore use the value or values of the extension field or at least aportion of the values when decoding the encoded representation of theslice and in particular during decoding of the slice data in the encodedrepresentation.

The extension field could, for instance, include one or more flagsand/or other syntax elements that dictate and provide instructions tothe decoder of how to perform the decoding of the slice data. Hence, theextension-compliant decoder makes use of the data in the extension fieldin the slice header during the parsing and decoding process, andoptionally including post-decoding processing of the slice data.

Decoding of the encoded representation of the slice based on at leastone value of the extension field in step S82 relates to any processingof the encoded representation of the slice as performed by a decoder.Hence, decoding as defined in step S82 is not merely limited to theactual generation of pixel data of the slice from the slice data presentin the encoded representation of the slice. This means that the at leastone value of the extension field could, for instance, be used by thedecoder for handling a reference picture buffer comprising previouslydecoded pictures of a video sequence that are temporarily stored forbeing output, e.g. for display, and/or temporarily stored for being usedas decoding reference for following, according to a decoding order,pictures of the video sequence. The at least one value of the extensionfield could alternatively, for instance, be used by the decoder toremove or drop specific NAL units of the bitstream. For example, theextension field could indicate that a specific NAL unit can be dropped,and therefore not entered in or removed from the reference picturebuffer, if only a higher-layer encoded representation is being decoded.

Thus, the decoding as defined in step S82 encompasses various operationsof the decoder taking place in connection with, i.e. preceding,simultaneously or following, the actual generation of pixel values for apresent slice.

A particular embodiment relates to a method performed during decoding anencoded representation of a slice of a picture in a video sequencecomprising multiple pictures. Each picture of the video sequence belongsto a layer of multiple layers. The encoded representation comprises aslice header and slice data. The method comprises parsing a lengthindicator associated with the encoded representation and indicative of alength of an extension field in the slice header. The extension field inthe slice header is identified based on the length indicator. The methodalso comprises processing a layered structure of said multiple picturesbased on at least one value of the extension field in the slice header.

In this particular embodiment the pictures of the video sequence areorganized in a layered structure with the multiple layers. These layerscould then correspond to different views in the case of multi-view view,different temporal layers in the case of scalable video, etc. Theprocessing of the layered structure, including for instance removing oneor more pictures or NAL units carrying pictures, in the decoder is thenpreferably performed at least partly based on at least one value of theextension field.

Herein follows a number of example embodiments disclosing variants andalternatives of implementing the present embodiments.

In a first example embodiment there is a codeword (syntax element), i.e.the length indicator 23, in the slice header 21 that represents thelength of an extension field 24 in the slice header 21. The encoder isconfigured to insert the length indicator 23 as a syntax elementaccording to this first example embodiment. A decoder shall parse thevalue of the codeword, i.e. length indicator 23, in order to locate thestart of the slice data 25 and thereby skip the extension field 24.

The name of the codeword that represents the length of the extensionfield 24 could for example be slice_header_extension_length.

According to this example embodiment, the decoder is configured toperform the following steps.

-   -   1. The slice_header_extension_length syntax element 23 is        parsed.    -   2. The decoder parses and ignores the values of        slice_header_extension_length number of bits following directly        after the slice_header_extension_length syntax element 23.    -   3. The decoder parses and decodes the slice data 25 starting        with the first bit that follows after the parsing of        slice_header_extension_length number of bits (at the bit        position where the parsing in step 2 ended).

In this embodiment the decoder therefore parses and reads the data orvalues of the extension field 24 in the slice header 21 during theheader decoding. However, even if the values are parsed and possiblydecoded the decoder ignores the values, i.e. does not use them furtherduring the decoding and processing of the slice data 25.

Alternatively a decoder may be configured to perform the followingsteps:

-   -   1. The slice_header_extension_length syntax element 23 is        parsed.    -   2. The decoder jumps forward slice_header_extension_length        number of bits in the bitstream 20.    -   3. The decoder parses and decodes the slice data 25 starting        with the first bit that follows after the jump of        slice_header_extension_length number of bits (at the bit        position where the jump in step 2 ended).

In this embodiment the decoder does not parse or read the data or valuesof the extension field 24 but rather jumps over the portion of the sliceheader 21 that is occupied by the extension field 24 as defined based onthe length indicator 23.

In a second example embodiment, the presence of the syntax element 23described in the first example embodiment is conditioned on anothersyntax element, for example called slice_header_extension_present_flag.If slice_header_extension_present_flag is 0 theslice_header_extension_length syntax element 23 and the extension data24 is not present. If slice_header_extension_present_flag is 1 theslice_header_extension_length syntax element 23 and the extension data24 is present. The slice_header_extension_present_flag is preferablysignaled in the SPS but may alternatively be signaled in the PPS, VPS,APS or in the slice header 21.

Accordingly, the encoder is configured to condition the presence of thelength indicator 23 of the first example embodiment by using a flag.

According to this embodiment, the decoder is configured to perform thefollowing ordered steps:

-   -   1. The slice_header_extension_present_flag from the active SPS        is used.    -   2. If slice_header_extension_present_flag is equal to 1:        -   a. The slice_header_extension_length syntax element 23 is            parsed.        -   b. The decoder parses and ignores the values of            slice_header_extension_length number of bits following            directly after the slice_header_extension_length syntax            element 23.        -   c. The decoder parses and decodes the slice data 25 starting            with the first bit that follows after the parsing of            slice_header_extension_length number of bits (at the bit            position where the parsing in step b ended).    -   3. Otherwise, (slice_header_extension_present_flag is equal to        0), the decoder parses and decodes the slice data 25 without        parsing a slice_header_extension_length syntax element 23 and        without parsing any extension data 24. Neither the        slice_header_extension_length syntax element 23 nor extension        data 24 will be present in the slice.

Alternatively, a decoder may be configured to perform the followingsteps:

-   -   1. The slice_header_extension_present_flag from the active SPS        is used.    -   2. If slice_header_extension_present_flag is equal to 1:        -   a. The slice_header_extension_length syntax element 23 is            parsed.        -   b. The decoder jumps forward slice_header_extension_length            number of bits in the bitstream 20.        -   c. The decoder parses and decodes the slice data starting            with the first bit that follows after the jump of            slice_header_extension_length number of bits (at the bit            position where the jump in step b ended).

Otherwise, (slice_header_extension_present_flag is equal to 0), decoderparses and decodes the slice data 25 without parsing aslice_header_extension_length syntax element 23 and without performing ajump in the bitstream 20. Neither the slice_header_extension_lengthsyntax element 23 nor extension data 24 will be present in the slice.

The syntax table could for example look like this:

slice_header( ) Descriptor . . . if( slice_header_extension_present_flag) { slice_header_extension_length ue(v) for( i = 0; i <slice_header_extension_length; i++) slice_header_extension_data_flagu(1) } }

In a third example embodiment, the length of the entire slice header 21is given by a syntax element 23 and the extension field 24 is added atthe end of the slice header 21 similar to the extension fields of theparameter sets.

Accordingly, the encoder is configured to indicate the length of theentire slice header 21 by a syntax element, i.e. the length indicator23, and the extension field 24 is added at the end of the slice header21 similar to the extension fields of the parameter sets.

According to this embodiment, the decoder is configured to perform thefollowing steps:

-   -   1. The slice_header_extension_present_flag from the active SPS        is used, if present. Otherwise steps 1, 2 and 3 are omitted and        only steps a, b and c are performed.    -   2. If slice_header_extension_present_flag is equal to 1:        -   a. The slice_header_extension_length is calculated as the            length of the slice header 21 minus the current bit            position.        -   b. The decoder parses and ignores the values of            slice_header_extension_length number of bits following            directly after the slice_header_extension_length syntax            element, i.e. the syntax element indicating the length of            the entire slice header 21.        -   c. The decoder parses and decodes the slice data 25 starting            with the first bit that follows after the parsing of            slice_header_extension_length number of bits (at the bit            position where the parsing in step b ended).    -   3. Otherwise, (slice_header_extension_present_flag is equal to        0), decoder parses and decodes the slice data 25 without parsing        a slice_header_extension_length syntax element 23 and without        parsing any extension data 24. Neither the        slice_header_extension_length syntax element 23 nor extension        data 24 will be present in the slice.

Thus, in this embodiment step S10 of FIG. 5 relating to thedetermination of the length of the extension field 24 is preferablyperformed by calculating the length of the extension field 24 bysubtracting a bit or byte position of an end of the length indicator 23in the slice header 21 from a value represented by the length indicator23. This value represented by the length indicator 23 is then preferablythe length of the slice header 21.

In a variant of step 3 of the first and second example embodiment andstep c of the third example embodiment byte alignment data 22 can beinterposed between the extension field 24 and the slice data 25. In sucha case, the decoder parses the byte alignment data 22 starting with thefirst bit that follows after the parsing ofslice_header_extension_length number of bits (first and third exampleembodiment) or the first bit that follows after the jump ofslice_header_extension_length number of bits (second exampleembodiment). The decoder then, once the end of the byte alignment data22 has been reached, parses and decodes the slice data 25.

In a fourth example embodiment a specific bit-pattern (“startcode”) isused to indicate the end of the extension data field 24. Hence, theencoder is configured to insert such a specific bit-pattern. The decoderparses all data until the bit-pattern is found. The bit-pattern may ormay not be required to be byte aligned. Then the decoder decodes theslice data 25 starting with the first bit after the specificbit-pattern.

A decoder may perform steps:

-   -   1. The slice header 21 is parsed up until the point where the        extension field 24 starts.    -   2. From that point the decoder scans the bitstream and searches        for the specific bit-pattern (the “start code”) that indicates        where the extension data 24 ends and the slice data 25 begins.    -   3. After the point where the specific bit-pattern is found the        decoder parses and decodes the slice.

Thus, in this embodiment the method as disclosed in FIG. 4 furthercomprises parsing the slice header 21 from a start of the slice header21 up until a start of the extension field 24 in the slice header 21.The method further comprises scanning the slice header 21 from the startof the extension field 24 until reaching the length indicator 23, whichis in the form of a specific start code. The method also comprisesidentifying an end of the extension field 24 as the specific start code.

In a fifth example embodiment any of first to third example embodimentsis used but with the difference that slice_header_extension_length 23 isnot signaled in the slice header 21. Instead it is signaled in a datastructure which can be referred to by the slice header 21, such as theSPS, VPS, PPS or APS.

In a sixth example embodiment any of the above presented exampleembodiments is used but with the difference that the length is expressedin bytes instead of bits.

In a seventh example embodiment any of second to sixth exampleembodiment is used with a bitstream requirement thatslice_header_extension_present_flag must be set equal to 0. An encodermay be configured to set the slice_header_extension_present_flag to 0.

In a particular embodiment, an encoder compliant to a certain profile,in particular the base profile, is preferably forced to setslice_header_extension_present_flag to 0. However, a decoder compliantto the same profile must use the length indicator and ignore theextension data.

In an eighth example embodiment any of second to sixth exampleembodiment is used with a specific meaning introduced for the extensiondata field 24. Such a meaning can correspond to additional informationin a multi-view extension or in a scalable extension.

An encoder may use the example embodiments by setting theslice_header_extension_present_flag to 1 and for each slice header 21encode values of the extension data field 23 according to the meaningthat has been introduced. The encoder is further configured to calculatethe length of the extension data field 24 and signal that in theslice_header_extension_length syntax element 23. Alternatively if thelength of the extension 24 is known a prior (e.g. a fixed lengthextension is used) the length can be signaled in theslice_header_extension_length syntax element 23 without firstcalculating it.

A decoder that recognizes (and makes use of) the introduced meaning forthe extension data 24 may be configured to perform the following steps:

-   -   1. The slice_header_extension_present_flag from the active SPS        is used.    -   2. If slice_header_extension_present_flag is equal to 1:        -   a. The slice_header_extension_length syntax element 23 is            parsed (or ignored if the extension length is already            known).        -   b. The decoder parses and uses the values of            slice_header_extension_length number of bits following            directly after the slice_header_extension_length syntax            element 23 according introduced meaning of the bits.        -   c. The decoder parses and decodes the slice data 25 starting            with the first bit that follows after the jump of            slice_header_extension_length number of bits (at the bit            position where the jump in step b ended).    -   3. Otherwise, (slice_header_extension_present_flag is equal to        0), decoder parses and decodes the slice data 25 without parsing        a slice_header_extension_length syntax element 23 and without        performing a jump in the bitstream 20. Neither the        slice_header_extension_length syntax element 23 nor extension        data 24 will be present in the slice.

A decoder that does not recognize (or does not make use of) theintroduced meaning for the extension data 24 may be configured toperform the steps presented in the second example embodiment.

In a ninth example embodiment, the eighth example embodiment is usedwith the specific meaning of the extension data field 24 tied to one ormore profiles of HEVC. That is, a decoder that decodes a bitstream 20that conforms to a profile that uses the extension data field 24 for aspecific purpose (with a specified meaning) will decode the extensiondata 24 as described in the eighth embodiment and use the extension data24 in accordance with the defined meaning.

A decoder that decodes a bitstream 20 that conforms to a profile thatdoes not use the extension data 24 for a specific purpose will performthe steps presented in the second example embodiment.

In a tenth example embodiment any of the second, third, eighth or ninthexample embodiment is used but where there is noslice_header_extension_present_flag, instead it is inferred to be 1. Noextension is signaled by signaling a zero extension length.

In an eleventh example embodiment any of the example embodiments abovecan be used but where the size of the extension 24 is not signaled.Instead a series of fixed and/or variable size variable length codes(VLCs) are defined.

In a twelfth example embodiment two slice header mechanisms could beused for a given slice. In this case, a first length indicator issignaled in a parameter set, such as PPS, whereas a second lengthindicator is signaled in the slice header. In an embodiment, a flag inthe PPS indicates whether the slice header comprises the second lengthindicator or not.

The PPS RBSP syntax may therefore look like:

pic_parameter_set_rbsp( ) { Descriptor . . .num_slice_header_extension_bits u(5) slice_header_extension_present_flagu(1) . . .

In this example, num_slice_header_extension_bits represents the firstlength indicator. A value of this first length indicator equal to 0specifies that no extra slice header bits are present in the sliceheader RBSP for coded pictures referring to the PPS. The syntax elementnum_slice_header_extension_bits should be equal to 0 for bitstreamsconforming to base profile specification. The flagslice_header_extension_present_flag is used to signal the presence ofthe second length indicator in the slice header. A value of 0 of thisflag specifies that no slice header extension syntax elements arepresent in the slice headers for coded pictures referring to the PPS.The flag slice_header_extension_present_flag should be equal to 0 forbitstreams conforming to base profile specification.

The slice layer RBSP syntax may then look like:

slice_layer_rbsp( ) { slice_header( ) slice_data( )rbsp_slice_trailing_bits( ) }

Correspondingly, the slice header syntax would then be as defined below:

slice_header( ) { Descriptor . . . for( i = 0; i <num_slice_header_extension_bits; i++ ) slice_header_extension_data_flagu(1) . . . if( slice_header_extension_present_flag ) {slice_header_extension_length ue(v) for( i = 0; i <slice_header_extension_length; i++ ) slice_header_extension_data_byteu(8) } byte_alignment( ) }

The first extension field in the slice header is, in this example, madeup of the slice_header_extension_data_flag. The second length indicator,slice_header_extension_length, specifies the length of the secondextension field, i.e. slice_header_extension_data_byte, not includingthe bits used for signaling slice_header_extension_length itself. In anexample, the value of slice_header_extension_length could be in therange of 0 to 256, inclusive. The second extension field in the sliceheader may have any value.

A thirteenth example embodiment is similar to the twelfths exampleembodiment disclosed above but with the following slice layer RBSPsyntax.

slice_layer_rbsp( ) { Descriptor slice_header( ) slice_data( ) if(slice_header_extension_present_flag ) { slice_extension_flag u(1) if(slice_extension_flag ) while( more_rbsp_data( ) )slice_extension_data_flag u(1) } rbsp_slice_trailing_bits( ) }

In an example implementation of the twelfth and the thirteenth, the basespecification could look like:

for( i = 0; i < num_slice_header_extension_bits; i++ )slice_header_extension_data_flag

An extension specification may the do this:

inter_layer_prediction_flag

inter_layer_prediction_flag equal to 0 indicates that the pictureassociated with the NAL unit is not used for reference in any picturebelonging to a higher spatial or multi-view layer.inter_layer_prediction_flag equal to 1 indicates that the pictureassociated with the NAL unit may be used for reference in a picturebelonging to a higher spatial or multi-view layer.

Thus, in this example implementation the first extension field in theslice header could be interpreted, by a decoder conforming to theextension specification, as indicating inter_layer_prediction_flag.

In an implementation embodiment, the extension field or extension fieldsin the slice header are interpreted as or replaced by, in the extensionspecification, actual codewords instead of the extension flag, forinstance and with reference to the twelfth and thirteenth exampleembodiment, slice_header_extension_data_flag andslice_header_extension_data_byte. The codewords may then be in the formof flags, fixed length code (FLC) words, variable length code (VLC)words, or even a mix of them.

It is also possible that an extension-compliant decoder ignores some ofthe bits in the extension field. For instance, it may the case that thecodewords of an extension field would sum up to 12 bits but the totallength of the extension field as signaled by the length indicator is setto 14 bits. Then the two remaining bits are simply ignored by theextension compliant decoder.

As mentioned in the foregoing, the extension field 24 could, in anexample, comprise encoded multi-view extension data and/or encodedscalable extension data.

In former case, the video stream is a stream of pictures from multiple(camera) views, for instance a right camera view and a left camera viewin the case of stereoscopic video. In such a case, one of thesemultiple, i.e. at least two, views could be a base view with the one ormore additional views encoded relative to the base view and possiblyeach other. However, the base view is preferably independentlydecodable, i.e. the base view is not using pictures in other views forreference. In such a case, a legacy decoder could correctly decode thebase view even if there is extension data relating to the additionalviews or to the multi-view structure present in the slice headers. Thisis possible due to the length indicator as used according to the variousembodiments as disclosed herein.

In the latter case, the video stream is a stream of pictures frommultiple layers. In such a case, one of these multiple layers is thebase layer with one or more additional layers. The base layer is thenpreferably independently decodable, i.e. without reference to picturesin any of the additional layers. Correspondingly, a picture belonging toadditional layer N can preferably reference pictures in the base layerand in layers up to but not above layer N. In such a case, a legacydecoder could correctly decode the base layer even if there is extensiondata relating to the additional layers or to the multilayer structurepresent in the slice headers. This is possible due to the lengthindicator as used according to the various embodiments as disclosedherein.

The embodiments are not limited to HEVC but may be applied to anyextension of HEVC, such as a scalable extension or multi-view extension,or to a different video codec, or indeed any other video codingstandard.

FIG. 9 is a schematic flow diagram of a method of encoding a slice 3 ofa picture 2, such as a picture 2 in a video stream 1. The methodcomprises generating a length indicator 23 in step S40. This lengthindicator 23 is indicative of a length of an extension field 24 presentin a slice header 21 of the slice 3. In a next step S41 the slice 3 isencoded into an encoded representation 20 of the slice 3 comprising theslice header 21 and slice data 25. The length indicator 23 is associatedwith or to the encoded representation 20 in step S42. Step S42 may beperformed prior to, substantially in parallel with or after step S41.

The encoded representation 20 may be further processed to form a NALunit 11, which optionally may be packaged into a data packet 10 with oneor more headers 12 as shown in FIG. 2.

If the slice header is to contain multiple extension fields steps S40and S42 of FIG. 9 are preferably once for each such extension field.

FIG. 10 is a flow diagram illustrating additional, optional steps of themethod in FIG. 9. In step S51 a length of the extension field 24 presentin the slice header 21 is determined. The length indicator 23 is thengenerated in step S40 of FIG. 9 based on the length of the extensionfield 24. Hence, in a particular embodiment the length indicator 23 isgiven a value that represents the length of the extension field 24 inbytes or bits.

In an optional step S50 the values of the extension field 24 isdetermined in order to see how long the extension field 24 becomes.Alternatively, the extension field 24 could, depending on what type ofextension data, such as multi-view extension data or scalable extensiondata, have a predefined length. As mentioned previously the values ofthe extension field 24 preferably represent multi-view extension dataand/or scalable extension data for the slice 3.

The length indicator 23 generated in step S40 is preferably insertedinto the slice header 21 in step S42. Alternatively, the lengthindicator 23 could be inserted into a parameter set that applies to thecurrent slice 3. In such a case, the slice header 21 preferablycomprises a parameter set identifier enabling identification of therelevant parameter set that comprises the length indicator 23.

If the length indicator 23 is inserted into the slice header 21 in stepS42 the length indicator 23 is preferably inserted directly precedingthe extension field 24 in the slice header 21.

In an embodiment, a slice header extension present flag is used tosignal the presence or absence of an extension field 24 in a sliceheader 21. This embodiment is schematically illustrated in FIG. 11. Themethod starts in step S60 where the slice header extension present flagis set to a predefined value, such as 1_(bin), to indicate presence ofthe extension field 24 in the slice header 21. In a next step S61 theslice header extension present flag is associated with the encodedrepresentation 20. This association of step S61 could be the insertionof the slice header extension present flag in a parameter set that isapplicable to the current slice 3. In such a case, the slice header 21preferably comprises a parameter set identifier enabling identificationof the relevant parameter set that comprises the length indicator 23.

FIG. 12 illustrates a particular embodiment of the associating step S61in FIG. 11. In this embodiment the slice header extension present flagis inserted into a parameter set, e.g. a PPS in step S70. A parameterset identifier 26 identifying the parameter set comprising the sliceheader extension present flag is preferably associated with the encodedrepresentation 20 in step S71, such as by inserting the parameter setidentifier 26 into the slice header 21 of the encoded representation 20.

In an alternative approach the slice header extension present flag isinserted into the slice header 21.

FIG. 13 is a schematic block diagram of a decoder 40 according to anembodiment. The decoder 40 comprises a parsing unit 41, also denotedparser or parsing means or module, configured to parse a lengthindicator associated with an encoded representation of a slice of apicture. The length indicator is indicative of a length of an extensionfield present in a slice header of the encoded representation. Thedecoder 40 also comprises a processing unit 42, also denoted processoror processing means or module, configured to cause the decoder 40 toignore, during decoding of the encoded representation, any value of theextension field identified based on the length indicator.

In an optional embodiment, the decoder 40 also comprises a determiningunit 43, also denoted determiner or determining means or module,configured to determine a length of the extension field based on thelength indicator. An optional identifying unit 44, also denotedidentifier or identifying means or module, of the decoder 40 ispreferably configured to identify an end of the extension field in theslice header based on the length as determined by the determining unit43.

The parsing unit 41 is preferably configured to parse the lengthindicator present in the slice header of the encoded representation.Alternatively, the parsing unit 41 could parse the length indicator in aparameter set identified based on a parameter set identifier parsed inthe slice header of the encoded representation. In this latter case, theparsing unit 41 could, if the parameter set with the length indicatoralready has been parsed and decoded for a previously encodedrepresentation, retrieve the length indicator form an associated memoryas previously disclosed herein.

In an optional embodiment the processing unit 42 is configured to causethe decoder 40 to ignore, during decoding of the encoded representation,any value of the extension field directly following the length indicatorin the slice header and up to an end of the extension field asidentified based on the length indicator, for instance by theidentifying unit 44.

The parsing unit 41 is preferably configured also to parse a sliceheader extension flag associated with the encoded representation. Insuch a case, the parsing unit 41 is configured to parse the lengthindicator and the processing unit 42 is configured to cause the decoder40 to ignore any value of the extension field if the slice headerextension flag has a predefined value, preferably 1_(bin).

In an optional embodiment the decoder 40 further comprises a setidentifying unit 45, also denoted set identifier or set identifyingmeans or module, configured to identify a parameter set, preferably aPPS, associated with the encoded representation based on a parameter setidentifier, preferably a PPS identifier, obtainable based on the sliceheader. In a particular embodiment, the set identifying unit 45preferably retrieves the parameter set identifier from the slice headerof the encoded representation and uses it to identify the relevantparameter set carrying the slice header extension present flag. In sucha case, the parsing unit 41 is configured to parse the slice headerextension present flag present in the parameter set, preferably PPS, asidentified by the set identifying unit 45.

The previously mentioned identifying unit 44 is in an optionalembodiment configured to identify a start of slice data in the encodedrepresentation based on an end of the extension field as identifiedbased on the length indicator, for instance by the identifying unit 44.The decoder 40 then comprises a decoding unit 46, also denoted datadecoder or decoding means or module, configured to parse and decode theslice data starting from the start of the slice data as identified bythe identifying unit 44.

The decoding unit 46 is preferably configured to parse and decode theslice data by ignoring and thereby not using any value of the extensionfield in the slice header. Hence, the decoding unit 46 performs thedecoding of the encoded representation 20 without using any value in orrepresented by the extension field.

In a particular embodiment the decoder 40 comprises a unit 41 forparsing a flag indicating whether a length indicator for indicating thelength of the slice header is used. Further, the decoder 40 comprises aunit 41 for parsing the length indicator and based on that determine thelength of an extension field in the slice header. The decoder 40 canthen use that information when decoding the encoded NAL unit.

FIG. 14 is a schematic block diagram of a decoder 90 according toanother embodiment for decoding an encoded representation of a slice ofa picture. The decoder 90 comprises a parsing unit 91, also denotedparser or parsing means or module, configured to parse a lengthindicator associated with the encoded representation. The lengthindicator is indicative of a length of an extension field present in aslice header of the encoded representation. An extension fieldidentifying unit 92, also denoted extension field identifier oridentifying means or module, is configured to identify the extensionfield in the slice header based on the length indicator parsed by theparsing unit 91. The decoder 90 also comprises a decoding unit 93, alsodenoted data decoder or decoding means or module. This decoding unit 93is configured to decode the encoded representation of the slice based onat least one value of the extension field in the slice header identifiedby the extension field identifying unit 92.

The decoders 40, 90 of FIGS. 13 and 14 with their including units 41-46,91-93 could be implemented in hardware. There are numerous variants ofcircuitry elements that can be used and combined to achieve thefunctions of the units 41-46, 91-93 of the decoder 40, 90. Such variantsare encompassed by the embodiments. Particular examples of hardwareimplementation of the decoder 40, 90 is implementation in digital signalprocessor (DSP) hardware and integrated circuit technology, includingboth general-purpose electronic circuitry and application-specificcircuitry.

The decoder 50 described herein could alternatively be implemented e.g.by one or more of a processor 52 and adequate software with suitablestorage or memory 54 therefore, a programmable logic device (PLD) orother electronic component(s) as shown in FIG. 15. In addition, thedecoder 50 preferably comprises an input or input unit 51 configured toreceive the encoded representations of slices, such as in the form ofNAL units. A corresponding output or output unit 53 is configured tooutput the decoded slices.

The decoder 32 can, for example, be located in a receiver 30, such as ina video camera, set-top-box or a display, e.g. in a mobile device asshown in FIG. 16. The receiver 30 then comprises an input or input unit31 configured to receive a coded bitstream, such as data packets of NALunits as shown in FIG. 2. The encoded representations of the NAL unitsare decoded by the decoder 32 as disclosed herein. The decoder 32preferably comprises or is connected to a reference picture buffer 34that temporarily stores already decoded pictures that are to be used asreference pictures for other pictures in the video stream. Decodedpictures are output from the receiver 30, such as from the referencepicture buffer 34, by means of an output or output unit 33. These outputpictures are sent to be displayed to a user on a screen or display of orconnected, including wirelessly connected, to the receiver 30.

An embodiment therefore relates to a receiver 30 comprising a decoderfor decoding an encoded representation of a slice of a picture. Theencoded representation comprises a slice header and slice data. Thedecoder of the receiver comprises a parsing unit configured to parse alength indicator associated with the encoded representation andindicative of a length of an extension field in the slice header. Thedecoder also comprises a processing unit configured to cause the decoderto ignore, during, decoding of the encoded representation, any value ofthe extension field identified based on the length indicator.

Another embodiment relates to a receiver 30 comprising a decoder fordecoding an encoded representation of a slice of a picture. The encodedrepresentation comprises a slice header and slice data. The decoder ofthe receiver comprises a parsing unit configured to parse a lengthindicator associated with the encoded representation and indicative of alength of an extension field in the slice header. The decoder alsocomprises an extension field identifying unit configured to identify theextension field in the slice header based on the length indicator. Adecoding unit of the decoder is configured to decode the encodedrepresentation of the slice based on at least one value of the extensionfield in the slice header identified by the extension field identifyingunit.

FIG. 17 is a schematic block diagram of an encoder 70 configured toencode a slice of a picture according to an embodiment. The encoder 70comprises a generating unit 71, also denoted generator or generatingmeans or module, configured to generate a length indicator indicative ofa length of an extension field present in a slice header of the slice.An encoding unit 72, also denoted slice encoder or encoding means ormodule, of the encoder 70 is configured to encode the slice into anencoded representation of the slice comprising the slice header andslice data. The encoder 70 also comprises an associating unit 73, alsodenoted associator or associating means or module, configured toassociate the length indicator with the encoded representation.

In an optional embodiment the encoder 70 comprises a determining unit74, also denoted determiner or determining means or module, configuredto determine a length of the extension field present in the sliceheader. The generating unit 71 is then configured to generate the lengthindicator based on the length of the extension field as determined bythe determining unit 74.

An optional embodiment has an associating unit 73 that is configured toinsert the length indicator in the slice header, preferably prior to theencoding unit 72 encoding the slice. In such a case, the associatingunit 73 preferably inserts the length indicator directly preceding theextension field in the slice header.

The encoder 70 comprises in an optional embodiment a setting unit 75,also denoted setter or setting means or module, configured to set aslice header extension present flag to a predefined value, preferably1_(bin), to indicate the presence of the extension field in the sliceheader. The associating unit 73 is then configured to associate theslice header extension present flag to the encoded representation. In aparticular embodiment, the associating unit 73 is configured to insertthe slice header extension present flag in a parameter set, preferablyPPS. The associating unit 73 is further configured to associate aparameter set identifier, preferably PPS identifier, identifying theparameter set, preferably PPS, with the encoded representation. Inparticular, the associating unit 73 is configured to insert theparameter set identifier, preferably PPS identifier, into the sliceheader.

In a particular embodiment, the encoder 70 comprises a unit 74 fordetermining the length of an extension field in the slice header and aunit 73 for inserting a length indicator.

The encoder 70 of FIG. 17 with its including units 71-75 could beimplemented in hardware. There are numerous variants of circuitryelements that can be used and combined to achieve the functions of theunits 71-75 of the encoder 70. Such variants are encompassed by theembodiments. Particular examples of hardware implementation of theencoder 70 is implementation in digital signal processor (DSP) hardwareand integrated circuit technology, including both general-purposeelectronic circuitry and application-specific circuitry.

The encoder 80 described herein could alternatively be implemented e.g.by one or more of a processor 82 and adequate software with suitablestorage or memory 84 therefore, a programmable logic device (PLD) orother electronic component(s) as shown in FIG. 18. In addition, theencoder 80 preferably comprises an input or input unit 81 configured toreceive the pictures of the video stream. A corresponding output oroutput unit 83 is configured to output the encoded representations ofthe slices, preferably in the form of NAL units.

The encoder 62 can, for example, be located in a transmitter 60 in avideo camera e.g. in a mobile device as shown in FIG. 19. Thetransmitter 60 then comprises an input or input unit 61 configured toreceive pictures of a video stream to be encoded. The pictures areencoded by the encoder 62 as disclosed herein. Encoded pictures areoutput from the transmitter 60 by an output or output unit 63 in theform of a coded bitstream, such as of NAL units or data packets carryingsuch NAL units as shown in FIG. 2.

An embodiment relates to a transmitter 60 comprising an encoder forencoding a slice of a picture. The encoder comprises a generating unitconfigured to generate a length indicator indicative of a length of anextension field present in the slice header of the slice. The encoderalso comprises an encoding unit configured to encode the slice into anencoded representation of the slice comprising the slice header andslice data. An associating unit of the encoder is configured toassociate the length indicator with the encoded representation.

The embodiments apply to a decoder, an encoder and any element thatoperates on a bitstream, such as a network-node or a Media Aware NetworkElement.

It is to be understood that the choice of interacting units or modules,as well as the naming of the units are only for exemplary purpose, andmay be configured in a plurality of alternative ways in order to be ableto execute the disclosed process actions.

It should also be noted that the units or modules described in thisdisclosure are to be regarded as logical entities and not with necessityas separate physical entities. It will be appreciated that the scope ofthe technology disclosed herein fully encompasses other embodimentswhich may become obvious to those skilled in the art, and that the scopeof this disclosure is accordingly not to be limited.

Reference to an element in the singular is not intended to mean “one andonly one” unless explicitly so stated, but rather “one or more.” Allstructural and functional equivalents to the elements of theabove-described embodiments that are known to those of ordinary skill inthe art are expressly incorporated herein by reference and are intendedto be encompassed hereby. Moreover, it is not necessary for a device ormethod to address each and every problem sought to be solved by thetechnology disclosed herein, for it to be encompassed hereby.

In the preceding description, for purposes of explanation and notlimitation, specific details are set forth such as particulararchitectures, interfaces, techniques, etc. in order to provide athorough understanding of the disclosed technology. However, it will beapparent to those skilled in the art that the disclosed technology maybe practiced in other embodiments and/or combinations of embodimentsthat depart from these specific details. That is, those skilled in theart will be able to devise various arrangements which, although notexplicitly described or shown herein, embody the principles of thedisclosed technology. In some instances, detailed descriptions ofwell-known devices, circuits, and methods are omitted so as not toobscure the description of the disclosed technology with unnecessarydetail. All statements herein reciting principles, aspects, andembodiments of the disclosed technology, as well as specific examplesthereof, are intended to encompass both structural and functionalequivalents thereof. Additionally, it is intended that such equivalentsinclude both currently known equivalents as well as equivalentsdeveloped in the future, e.g. any elements developed that perform thesame function, regardless of structure.

Thus, for example, it will be appreciated by those skilled in the artthat block diagrams herein can represent conceptual views ofillustrative circuitry or other functional units embodying theprinciples of the technology. Similarly, it will be appreciated that anyflow charts, state transition diagrams, pseudo code, and the likerepresent various processes which may be substantially represented incomputer readable medium and so executed by a computer or processor,whether or not such computer or processor is explicitly shown.

The functions of the various elements including functional blocks may beprovided through the use of hardware such as circuit hardware and/orhardware capable of executing software in the form of coded instructionsstored on computer readable medium. Thus, such functions and illustratedfunctional blocks are to be understood as being eitherhardware-implemented and/or computer-implemented, and thusmachine-implemented.

FIG. 20 is a schematic illustration of a computer 100 comprising aprocessor 110 and a computer readable medium 120 having stored therein acomputer program 130.

In an embodiment, the computer program 130 is configured for decoding anencoded representation of a slice of a picture. The encodedrepresentation comprises a slice header and slice data. The computerprogram 130 comprises code means which when run by the processor 110causes the processor 110 to parse a length indicator associated with theencoded representation and indicative of a length of an extension fieldin the slice header. The processor 110 is also caused to determine toignore, during decoding of the encoded representation, any value of saidextension field identified based on the length indicator.

In another embodiment, the computer program 130 is configured fordecoding an encoded representation of a slice of a picture. The encodedrepresentation comprises a slice header and slice data. The computerprogram 130 comprises code means which when run by the processor 110causes the processor 110 to parse a length indicator associated with theencoded representation and indicative of a length of an extension fieldin the slice header. The processor 110 is also caused to identify, basedon the length indicator, the extension field in said slice header anddecode the encoded representation of the slice based on at least onevalue of the extension field identified based on the length indicator.

In a further embodiment, the computer program 130 is configured forencoding a slice of a picture. The computer program 130 comprises codemeans which when run by the processor 110 causes the processor 110 togenerate a length indicator indicative of a length of an extension fieldpresent in a slice header of the slice. The processor 110 is also causedto encode the slice into an encoded representation of the slicecomprising the slice header and slice data. The processor 110 is furthercaused to associate the length indicator with the encodedrepresentation.

An embodiment relates to a computer program product comprising computerreadable medium 120 and a computer program 130 as defined according toany of the above disclosed embodiments stored on the computer readablemedium 120.

In terms of hardware implementation, the functional blocks may includeor encompass, without limitation, digital signal processor (DSP)hardware, reduced instruction set processor, hardware (e.g., digital oranalog) circuitry including but not limited to application specificintegrated circuit(s) (ASIC), and (where appropriate) state machinescapable of performing such functions.

The embodiments described above are to be understood as a fewillustrative examples of the present invention. It will be understood bythose skilled in the art that various modifications, combinations andchanges may be made to the embodiments without departing from the scopeof the present invention. In particular, different part solutions in thedifferent embodiments can be combined in other configurations, wheretechnically possible. The scope of the present invention is, however,defined by the appended claims.

The invention claimed is:
 1. A method of decoding an encodedrepresentation of a slice of a picture, said encoded representationcomprising a slice header and slice data, said method comprising:parsing a length indicator associated with said encoded representationand indicative of a length of an extension field in said slice header;and determining to ignore, during decoding of said encodedrepresentation of said slice of said picture, any value of saidextension field identified based on said length indicator.
 2. The methodaccording to claim 1, further comprising: determining a length of saidextension field based on said length indicator; and identifying an endof said extension field in said slice header based on said length. 3.The method according to claim 1, wherein parsing said length indicatorcomprises parsing said length indicator present in said slice header. 4.The method according to claim 3, wherein determining to ignore said anyvalue comprises determining to ignore, during decoding of said encodedrepresentation, any value of said extension field directly followingsaid length indicator in said slice header up to an end of saidextension field as identified based on said length indicator.
 5. Themethod according to claim 1, wherein parsing said length indicatorcomprises parsing said length indicator present in a parameter setidentified based on a parameter set identifier obtainable based on saidslice header.
 6. The method according to claim 1, further comprising:parsing a slice header extension present flag associated with saidencoded representation; and performing said parsing of said lengthindicator and said determining to ignore said any value if said sliceheader extension present flag has a predefined value.
 7. The methodaccording to claim 6, further comprising: identifying a parameter setassociated with said encoded representation based on a parameter setidentifier obtainable based on said slice header, wherein parsing saidslice header extension present flag comprises parsing said slice headerextension present flag present in said parameter set.
 8. The methodaccording to claim 1, further comprising: identifying a start of saidslice data based on said length indicator; and parsing and decoding saidslice data starting from said start of said slice data.
 9. The methodaccording to claim 8, wherein parsing and decoding said slice datacomprises parsing and decoding said slice data by ignoring said anyvalue of said extension field in said slice header.
 10. A method ofdecoding an encoded representation of a slice of a picture, said encodedrepresentation comprising a slice header and slice data, said methodcomprising: parsing a length indicator associated with said encodedrepresentation of said slice of said picture and indicative of a lengthof an extension field in said slice header of said encodedrepresentation of said slice of said picture; identifying, based on saidlength indicator, said extension field in said slice header; anddecoding said encoded representation of said slice based on at least onevalue of said extension field identified based on said length indicator.11. A method of encoding a slice of a picture, said method comprising:generating a length indicator indicative of a length of an extensionfield present in a slice header of said slice; encoding said slice intoan encoded representation of said slice comprising said slice header andslice data; and associating said length indicator with said encodedrepresentation.
 12. The method according to claim 11, furthercomprising: determining a length of said extension field present in saidslice header, wherein generating said length indicator comprisesgenerating said length indicator based on said length of said extensionfield.
 13. The method according to claim 11, wherein associating saidlength indicator comprises inserting said length indicator into saidslice header.
 14. The method according to claim 13, wherein insertingsaid length indicator comprises inserting said length indicator directlypreceding said extension field in said slice header, and said lengthindicator consists of an encoded length value that specifies the lengthof said extension field.
 15. The method according to claim 11, whereinassociating said length indicator comprises inserting said lengthindicator into a parameter set and associating a parameter setidentifier identifying said parameter set with said encodedrepresentation.
 16. The method according to claim 11, furthercomprising: setting a slice header extension present flag to apredefined value to indicate presence of said extension field in saidslice header; and associating said slice header extension present flagwith said encoded representation.
 17. The method according to claim 16,wherein associating said slice header extension present flag comprises:inserting said slice header extension present flag into a parameter set;and associating a parameter set identifier identifying said parameterset with said encoded representation.
 18. A decoder for decoding anencoded representation of a slice of a picture, said encodedrepresentation comprising a slice header and slice data, said decodercomprising: a parsing unit configured to parse a length indicatorassociated with said encoded representation and indicative of a lengthof an extension field in said slice header; and a processing unitconfigured to cause said decoder to ignore, during decoding of saidencoded representation, any value of said extension field identifiedbased on said length indicator.
 19. The decoder according to claim 18,further comprising: a determining unit configured to determine a lengthof said extension field based on said length indicator; and anidentifying unit configured to identify an end of said extension fieldin said slice header based on said length.
 20. The decoder according toclaim 18, wherein said parsing unit is configured to parse said lengthindicator present in said slice header.
 21. The decoder according toclaim 20, wherein said processing unit is configured to cause saiddecoder to ignore, during decoding of said encoded representation, anyvalue of said extension field directly following said length indicatorin said slice header up to an end of said extension field as identifiedbased on said length indicator.
 22. The decoder according to claim 18,wherein said parsing unit is configured to parse said length indicatorpresent in a parameter set identified based on a parameter setidentifier obtainable based on said slice header.
 23. The decoderaccording to claim 18, wherein said parsing unit is configured to parsea slice header extension present flag associated with said encodedrepresentation; and said parsing unit is configured to parse said lengthindicator and said processing unit is configured to cause said decoderto ignore said any value if said slice header extension present flag hasa predefined value.
 24. The decoder according to claim 23, furthercomprising: a set identifying unit configured to identify a parameterset associated with said encoded representation based on a parameter setidentifier obtainable based on said slice header, wherein said parsingunit is configured to parse said slice header extension present flagpresent in said parameter set.
 25. The decoder according to claim 18,further comprising: an identifying unit configured to identify a startof said slice data based on an end of said extension field as identifiedbased on length indicator; and a decoding unit configured to parse anddecode said slice data starting from said start of said slice data. 26.The decoder according to claim 25, wherein said decoding unit isconfigured to parse and decode said slice data by ignoring said anyvalue of said extension field in said slice header.
 27. A receivercomprising a decoder for decoding an encoded representation of a sliceof a picture, said encoded representation comprises a slice header andslice data, said decoder comprising: a parsing unit configured to parsea length indicator associated with said encoded representation andindicative of a length of an extension field in said slice header; and aprocessing unit configured to cause said decoder to ignore, during,decoding of said encoded representation, any value of said extensionfield identified based on said length indicator.
 28. A decoder fordecoding an encoded representation of a slice of a picture, said encodedrepresentation comprising a slice header and slice data, said decodercomprising: a parsing unit configured to parse a length indicatorassociated with said encoded representation and indicative of a lengthof an extension field in said slice header; an extension fieldidentifying unit configured to identify said extension field in saidslice header based on said length indicator; and a decoding unitconfigured to decode said encoded representation of said slice based onat least one value of said extension field identified by said extensionfield identifying unit.
 29. A receiver comprising a decoder for decodingan encoded representation of a slice of a picture, said encodedrepresentation comprises a slice header and slice data, said decodercomprising: a parsing unit configured to parse a length indicatorassociated with said encoded representation and indicative of a lengthof an extension field in said slice header; an extension fieldidentifying unit configured to identify said extension field in saidslice header based on said length indicator; and a decoding unitconfigured to decode said encoded representation of said slice based onat least one value of said extension field identified by said extensionfield identifying unit.
 30. An encoder for encoding a slice of apicture, said encoder comprising: a generating unit configured togenerate a length indicator indicative of a length of an extension fieldpresent in a slice header of said slice; an encoding unit configured toencode said slice into an encoded representation of said slicecomprising said slice header and slice data; and an associating unitconfigured to associate said length indicator with said encodedrepresentation.
 31. The encoder according to claim 30, furthercomprising: a determining unit configured to determine a length of saidextension field present in said slice header, wherein said generatingunit is configured to generate said length indicator based on saidlength of said extension field.
 32. The encoder according to claim 30,wherein said associating unit is configured to insert said lengthindicator into said slice header.
 33. The encoder according to claim 32,wherein said associating unit is configured to insert said lengthindicator directly preceding said extension field in said slice header.34. The encoder according to claim 30, wherein said associating unit isconfigured to insert said length indicator into a parameter set andassociating a parameter set identifier identifying said parameter setwith said encoded representation.
 35. The encoder according to claim 30,further comprising: a setting unit configured to set a slice headerextension present flag to a predefined value to indicate presence ofsaid extension field in said slice header, wherein said associating unitis configured to associate said slice header extension present flag withsaid encoded representation.
 36. The encoder according to claim 35,wherein associating unit is configured to insert said slice headerextension present flag into a parameter set and associate a parameterset identifier identifying said parameter set with said encodedrepresentation.
 37. A transmitter comprising an encoder for encoding aslice of a picture, said encoder comprising: a generating unitconfigured to generate a length indicator indicative of a length of anextension field present in a slice header of said slice an encoding unitconfigured to encode said slice into an encoded representation of saidslice comprising said slice header and slice data; and an associatingunit configured to associate said length indicator with said encodedrepresentation.
 38. A computer program product comprising anon-transitory computer readable medium storing a computer program fordecoding an encoded representation of a slice of a picture, said encodedrepresentation comprising a slice header and slice data, said computerprogram comprises code which when run by a processor causes saidprocessor to: parse a length indicator associated with said encodedrepresentation and indicative of a length of an extension field in saidslice header; and determine to ignore, during decoding of said encodedrepresentation, any value of said extension field identified based onsaid length indicator.
 39. A computer program product comprising anon-transitory computer readable medium storing a computer program fordecoding an encoded representation of a slice of a picture, said encodedrepresentation comprising a slice header and slice data, said computerprogram comprises code which when run by a processor causes saidprocessor to: parse a length indicator associated with said encodedrepresentation and indicative of a length of an extension field in saidslice header; identify, based on said length indicator, said extensionfield in said slice header; and decode said encoded representation ofsaid slice based on at least one value of said extension fieldidentified based on said length indicator.
 40. A computer programproduct comprising a non-transitory computer readable medium storing acomputer program for encoding a slice of a picture, said computerprogram comprises code which when run by a processor causes saidprocessor to: generate a length indicator indicative of a length of anextension field present in a slice header of said slice; encode saidslice into an encoded representation of said slice comprising said sliceheader and slice data; and associate said length indicator with saidencoded representation.
 41. The method of claim 1, further comprising:receiving a transport stream (TS) data packet comprising a header and apayload, wherein the payload comprises the slice header and the slicedata; extracting the slice header and the slice data from the TS datapacket.
 42. The method of claim 11, further comprising: after encodingsaid slice into an encoded representation of said slice comprising saidslice header and slice data, forming a transport stream (TS) data packetcomprising a header and a payload, wherein forming the TS data packetcomprises including the slice header and the slice data in the payloadof the TS data packet; and transmitting the TS data packet.
 43. Themethod according to claim 11, wherein associating said length indicatorcomprises inserting said length indicator into a parameter set, themethod further comprises inserting into the slice header a parameter setidentifier identifying said parameter set into which said lengthindicator is inserted, and no portion of the parameter set is containedin the slice header.
 44. A decoding method, comprising: obtaining anencoded representation of a slice of a picture, said encodedrepresentation comprising a slice header and slice data immediatelyfollowing the slice header, and said slice header comprising a variablelength header extension data field storing extension data; based oninformation associated with said encoded representation, determining anending position of the variable length header extension data field; andafter determining the ending position of the variable length headerextension data field, ignoring all of the extension data containedwithin the variable length header extension data field.
 45. The decodingmethod of claim 44, wherein said slice header further comprises avariable length header extension length field immediately preceding thevariable length header extension data field, said information associatedwith said encoded representation comprises an encoded length valuestored in the variable length header extension length field, wherein theencoded length value identifies X number of sequential bytes, X>0; andignoring all of the extension data contained within the variable lengthheader extension data field consists of ignoring the X number of bytesthat immediately follow the header extension length field.
 46. A decoderconfigured to perform the method of claim 45.